Alarm systems such as intrusion alarms, fire alarms and similar systems will age and in time ofter develop intermittent problems generating false alarms. False alarms may also be generated by various physical conditions related to the type of alarm system and more particularly to the type of sensors used in the system. Wind rattling a door, for example, may generate a momentary "false" sensor signal in an intrusion alarm system and trigger the alarm signalling device. Such intermittent "false" signals, which generate false alarms, are the bane of all practical alarm systems. Under practical circumstances, it is sometimes difficult or impossible to immediately resolve the physical or electrical problem which is causing the false alarm to be triggered. The problem is compounded where sensor elements are wired together to provide a signal when any one sensor element is triggered or malfunctions.
A person notified by the same alarm for the third or fourth time, which alarm has previously been determined to be a false alarm, is likely to be less than diligent in responding to that alarm, especially if there are other pressing matters to be attended to. It would therefore be advantageous, especially in a situation where the cause of the first alarm cannot be determined or corrected immediately, to have a means for disconnecting or disabling that portion or zone of the alarm system wherein the problem has repeatedly occurred, while still maintaining the operability and therefore the protection afforded by the remainder of the alarm system.
It is an object of the present invention to provide a control circuit for an alarm system, which control circuit will become insensitive to sensor signals received from sensors connected to it as a result of an alarm output signal generated by an alarm which has been switched from standby to an alarm mode, in response to the sensor signals.
Another purpose of this invention is to provide an alarm system having multiple zones or channels which will automatically become insensitive to repeated sensor signals from a particular zone while still maintaining responsiveness to sensor signals from the remaining zones.
It is a related purpose of this invention to provide a signalling means to automatically indicate which zones are operative and which zones are insensitive to sensor signals.
It is a further object of this invention to provide means for resetting those portions of the circuit which have become insensitive to sensor signals, utilizing manual or other signal generating means which are not controlled by the operation of the alarm signal.
Another object of this invention is to provide means for selectively disabling a particular zone or zones without activating the alarm signalling device, for use when it is known that a sensor will otherwise signal a false alarm.
It is a still further object of this invention to provide an input circuit having two modes and, depending on the mode, which will accept an input or sensor signal from either a normally open switch or circuit or a normally closed switch or circuit sensor device.
It is a still further object of this invention to provide an alarm control system which can be operated in conjunction with a redundant zone system in which two, or more, separate sensors respond to the same event, such as the opening of a particular door or window. If one of the sensors becomes imoperative because of a malfunction and its associated control circuit is therefore latched out, the second or redundant sensor is still in operation and will provide warning if the event occurs.
It is still a further object of this invention to provide a multi-zone alarm system which gives automatic continuing alarm capabilities by employing an alarm signalling device which automatically resets to its stand-by mode after a predetermined period of alarm signalling. The alarm signalling device can then continue to respond to operating signals from the non-latched zones.